Oxadiazolines cycloreversion

Cycloreversion of oxadiazolines (194) (with an anion stabilizing substituent in the 5-position) with a base affords amide anions, which can be intercepted with electrophiles El—to give (195) (Scheme 85). The sequence can be used to convert an aromatic aldehyde via the imine (193) into an amide (195) <91SL189>. 

Sometimes it is difficult to classify the fragmentation unequivocally, in particular when short-lived, reactive intermediates that readily decompose into smaller fragments may or may not be encountered depending on reaction conditions. As examples, the oxadiazolines (20,75) can be mentioned (see Sections V.A and VI). Many reactions classified as [5 - 2 + 2 + 1] fragmentations (Section VI) are probably initiated as a [5 —> 3 + 2] 1,3-dipolar cycloreversion. 

Three 3 + 2-cycloreversions of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazolines have been examined by DFT methods.8 The lowest activation energies have been found for cycloreversion to 2-diazopropane and acetic methylcarbonic anhydride and for cycloreversion to nitrogen and a carbonyl ylide. Those are the reactions that have also been observed experimentally. 

Lactones of azocarboxylic acids are remarkably reactive. In the presence of phenyl isocyanate, the imino isocyanate formed from reactive 2-hydrazono-A3-l,3,4-oxadiazolines via a 1,3-dipolar cycloreversion is intercepted to give [l,2,4]triazolo[l,2-a]-[l,2,4]triazole-l,3,5-triones by means of two subsequent [2 + 3] cycloadditions via azomethine imine intermediates (Scheme 10) (76T2685). 

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